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How do scientists use light to move matter?

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Scientists can now use optical tweezers, optical traps, and optical levitation to conduct delicate experiments. Optical.

The problem with modern biological science is that it deals with things that are unreasonably small. Gone are the days when being a biologist meant having a brush to cross-pollinate peas with, or a sketchbook to doodle pictures of iguanas in, or just a really good poking stick.* Sadly, the only thing that scientists learn when they poke things with sticks it that A.) some things go ‘squish' when they're B.) poked with sticks. There are only so many papers to be squeezed out of this.


That's where optical manipulation comes in. Using directed beams of light, scientists are able to trap, grip, and levitate small materials. We've featured articles on sonic levitation, but sound waves are caused and propagated by the physical manipulation of particles of matter. Sonic levitation was matter moving other matter. Optical manipulation is light moving matter. Photons haven't even been detected as having mass. However, they do have momentum. Momentum has to be conserved. When light hits matter it moves off in a slightly different direction. In order for momentum to be conserved, the matter that pushed the light in one direction gets a little kick in the other direction.


To manipulate matter the trick it to make sure that the matter is surrounded, on all sides, by light which will be deflected away from the center of the beam. If the matter moves slightly to one side of the beam, the light will get kicked outwards and will push the matter back to the center. Light doesn't just hold matter in place, it can also move it. Since the beam is deflected whether the matter moves closer to the edge of the light or the edge of the light moves closer to the matter, moving the light can gently, and precisely, move matter around.

Optical tweezers can manipulate organelles in a cell. They can be used to experiment on live bacteria, if they're in the infrared spectrum, since visible light will kill the bacteria. They can even be used in the physical sciences, which have also gotten distressingly small-oriented. Quantum mechanics is tough to observe because it is difficult to isolate particles from their environment enough for quantum behavior to be observable. Using light to isolate a small particle can allow scientists to cool that particle enough to get a look at some quantum weirdness.

It's unlikely that something big is going to be held in place by light alone. In order for the optical tweezers, traps and levitation to work, the object does have to be so small that the deflection of photons has a noticeable effect on it, so light-lassos aren't going on the market. No matter how cool they would look.


*In the old days, a good poke with a suitable stick would tell biologists things like ‘chimpanzees bite the hell out of anything that pokes them with sticks,' which in turn taught us important things about our own nature. Research like this is the primary reason why walking sticks fell out of favor and men started presenting the women they liked with only the kind of sticks that had flowers on the end of them – one of the major reasons for the population boom in the late forties and early fifties.

[Via, Stanford and The Scientist.]